The application of polymers to metal, particularly during the pipe forming process, is well known. Polymer liners are applied to the inner and/or outer surfaces of metal pipes to prevent corrosion and increase the durability and flow characteristics thereof.
Examples of processes for fabricating metal pipe with such a polymer liner are provided in U.S. Pat. No. 5,316,606 issued on May 31, 1994 to Andre and U.S. Ser. No. 08/225,440, filed on Apr. 8, 1994, the contents of both of which are hereby incorporated by reference.
A substantial problem associated with the extrusion of a polymer in such processes concerns what to do with the polymer when a stoppage of the metal pipe forming process occurs, such as happens during a mechanical failure or when the metal stock is depleted and must be replenished.
The polymer extrusion process cannot simply be halted during such stoppages because the polymer contained within the extruder screw and the piping intermediate the extruder screw and the extruder film die then would tend to overheat, and thus become unusable for subsequent application. Such overheating would occur because of the heat generated by the extruder screw and any external heaters, if utilized. It is not possible to merely stop the extruder screw and/or turn off any heaters since such action would result in undesirable hardening of the polymer within the extruder screw and pipes. Thus, the flow of polymer through the extruder screw and pipes must remain substantially continuous.
One solution is to direct the polymer to a repelletizing apparatus wherein the polymer is permitted to solidify and is then ground or repelletized such that it may subsequently be once again plasticized via the extruder screw and used in the extrusion process. However, such repelletizing apparatus are costly. Further, such reuse of the repelletized polymer involves the undesirable repetition of the melting and extruding processes. Such repetition is undesirable primarily because of the inefficiency in having to repeatedly melt the polymer. Further, such repetition causes undesirable and unnecessary wear and tear on the equipment utilized.
Additionally, the application of a desired thickness of polymer onto the surface of the metal pipe is substantially dependent upon maintenance of both the speed with which the metal is supplied to the pipe forming apparatus and the volume of polymer which is extruded through the polymer film die. This interdependency of the speed with which metal is provided and the extruded polymer volume makes it undesirable to allow changes in the extruder volume, such as by halting the flow of polymer during stoppage of the pipe forming process, for example. Such changes necessitate that the extruder volume be accurately readjusted to facilitate the resumed application of polymer to the metal pipe. Thus, it is undesirable to halt the flow of polymer from the extruder screw, since the desired flow rate would have to be re-established upon resuming the extruding process.
As such, it is desirable to provide an apparatus for diverting polymer flow from the extruder head such that polymer flow may be maintained during temporary stoppage of the pipe forming apparatus. It is further desirable to provide a desirable use for the polymer so diverted, such that the polymer need not be either repelletized or wasted.